GarfieldEr007

如何制作快速加载的HTML页面

以下技巧都是基于通用的知识和经验，如果你有可以帮助他人提高页面加载性能的其它方法，可以在本文讨论区提出。

一个好的页面不仅要给访客提供一个更有效的站点，同时也需要降低你的服务器压力和网络请求。后者对于那些高访问量的站点，或在有爆炸性新闻出现等特殊情况下会出现流量突增的站点来说尤为关键。

页面加载性能的优化不仅仅是针对那些带宽有限的拨号上网用户需要看的内容，对于提供给宽带用户访问的内容同样重要并且可以导致巨大的提升，哪怕对于那些拥有最快网速的访客。

Tips

减小页面的大小

在页面下载中，页面的大小至今扮演着非常重要的因素。

减小页面的大小能够通过排除不必要空格，注释，动态内嵌脚本，和放入外部文件的 CSS 等在页面结构中很小的改变都能够提高下载性能。

诸如HTML Tidy这类的工具可以从有效的HTML源文件中自动截去行首空格和额外的空行，其它工具则可以通过重新格式化源代码来压缩JavaScript或者通过混淆源码将长标识符替换为短标识符来减小文件大小。

最小化文件数量

减少一个页面引用的文件数量可以降低在下载一个页面的过程中需要的HTTP请求数量。

根据其缓存设置，浏览器可能会为每个 CSS/JavaScript/image 文件发送一个 If-Modified-Since 请求给网络服务器，以查询这些文件自上次加载后是否有被修改。

通过减少链接至网页的文件数量，你可以减少发送这些请求以及接收它们回应的时间。

在查询引用文件是否被修改上花费太多时间会延迟网页的初始化显示，因为在渲染页面之前浏览器必须确认每个 CSS 或 JavaScript 文件的修改时间。

减少域名查找

Since each separate domain costs time in a DNS lookup, reducing the number of separate domains used to reference CSS, JavaScript and images reduces page load times.

This may not always be practical; however, you should always be careful to only use the minimum number of different domains in your pages as possible.

缓存重用的内容

Make sure that any content that can be cached is cached with appropriate expiration times.

In particular pay attention to the Last-Modified header. It allows for efficient caching of pages; by means of this header, information is conveyed to the user agent as to when the file it wants to load has last been modified. For static pages (e.g. .html, .css), most web servers will already automatically append the Last-Modified header, based on the last modified date stored in the file system. For dynamic pages (e.g. .php, .aspx), this of course can't be done, and the header is not sent.

So, in particular for pages which are generated dynamically, a little research on this subject is beneficial. It can be somewhat involved, but it will save a lot of page requests on pages which would normally not be cacheable.

More information:

HTTP Conditional Get for RSS Hackers
HTTP 304: Not Modified
On HTTP Last-Modified and ETag

高效地排列页面组件

Download page content first so the user gets the quickest apparent response for page loading.

The content of the page along with any CSS or JavaScript required for its initial display should be downloaded first. This content is typically text and can benefit from text compression in the modem thus providing a quick response to the user.

Any DHTML features that require the page to complete loading before being used should be initially disabled and only enabled after the page loads. This will allow the DHTML JavaScript to be loaded after the page contents thus improving the overall appearance of the page load.

减少内联脚本的数量

Inline scripts can be expensive for page loading since the parser must assume that an inline script can modify the page structure. Reducing the use of inline scripts in general, and reducing the use of document.write() to output content in particular can improve overall page loading. Use modern W3C DOM methods to manipulate page content for modern browsers rather than the older approaches based on document.write().

使用现代CSS和合法标记

Use of modern CSS reduces the amount of markup, can reduce the need for images in terms of layout, and can in many ways replace images which are actually only images of text and cost much more than the equivalent CSS and text.

Using valid markup has other advantages. Not only do browsers not have to perform "error correction" when parsing the HTML, valid markup allows free use of other tools which canpre-process your web pages. For example, HTML Tidy can remove whitespace and remove optional ending tags, however, it will refuse to run on a page with serious markup errors.

给内容分块

Either replace table-based layout with

blocks or break tables into smaller tables that can be displayed without having to download the entire page contents.

Rather than deeply nesting tables as in:

<TABLE>
  <TABLE>
    <TABLE>
          ...
    TABLE>
  TABLE>
TABLE>

use unnested tables or divs as in

<TABLE>...TABLE>
<TABLE>...TABLE>
<TABLE>...TABLE>

指定图像和表格的大小

If the browser can immediately determine the height and/or width of your images and tables, it will be able to display a web page without having to reflow the content. This not only speeds the display of the page but prevent annoying changes in a page's layout when the page completes loading.

Images should have height and width specified.

Tables should use the CSS rule table-layout: fixed and specify widths of columns using COL and COLGROUP tags.

Choose your user agent requirements wisely

To achieve the greatest improvements in page design make sure that reasonable user agent requirements are specified for projects. Do not require your content to appear pixel-perfect in all browsers, especially not in downlevel browsers.

Ideally, your basic minimum requirements should be based upon modern browsers which support the relevant standards. This can include: Netscape 7/Gecko 1.0.2+ on any platform, Internet Explorer 5.5+ on Windows, Opera 7+ on Windows, and Safari on Mac OS X.

Note however that many of the tips listed in this Technote are common sense techniques which apply to any user agent and can be applied to any web page regardless of browser support requirements.

页面结构示例

· HTML

· HEAD

· LINK ...
CSS files required for page appearance. Minimize the number of files for performance while keeping unrelated CSS in separate files for maintenance.

· SCRIPT ...
JavaScript files for functions required during the loading of the page, but not any DHTML that can only run after page loads.

Minimize the number of files for performance while keeping unrelated JavaScript in separate files for maintenance.

· BODY

· User visible page content in small chunks (tables / divs) that can be displayed without waiting for the full page to download.

· SCRIPT ...
Any scripts which will be used to perform DHTML. DHTML script typically can only run after the page has completely loaded and all necessary objects have been initialized. There is no need to load these scripts before the page content. That only slows down the initial appearance of the page load.